1. Technical Field
The present invention relates to a surface acoustic wave resonator and a surface acoustic wave oscillator having the surface acoustic wave resonator, and more particularly, to a surface acoustic wave resonator in which grooves are formed in a substrate surface and a surface acoustic wave oscillator having the surface acoustic wave resonator.
2. Related Art
In a surface acoustic wave (SAW) device (such as a SAW resonator), a variation in frequency-temperature characteristic is greatly affected by a stop band of the SAW or a cut angle of a quartz crystal substrate, and a shape of an IDT (Interdigital Transducer).
For example, JP-A-11-214958 discloses a configuration for exciting an upper mode and a lower mode of a stop band of a SAW and a standing wave distribution in the upper mode and the lower mode of the stop band.
JP-A-2006-148622, JP-A-2007-208871, JP-A-2007-267033, and JP-A-2002-100959 disclose that the upper mode of the stop band has a frequency-temperature characteristic more excellent than that in the lower mode of the stop band of the SAW. JP-A-2006-148622 and JP-A-2007-208871 disclose that the cut angle of the quartz crystal substrate is adjusted and a normalized thickness (H/λ) of an electrode is increased to about 0.1 so as to obtain an excellent frequency-temperature characteristic in a SAW device using Rayleigh waves.
JP-A-2007-267033 discloses that the cut angle of the quartz crystal substrate is adjusted and a normalized thickness (H/λ) of an electrode is increased to about 0.045 or greater in a SAW device using Rayleigh waves.
JP-A-2002-100959 discloses that a rotational Y-cut X-propagation quartz crystal substrate is employed and that the frequency-temperature characteristic is more improved than that in the case where the resonance in the lower mode of the stop band is used, by using the resonance in the upper end of the stop band.
In a SAW device employing an ST-cut quartz crystal substrate, grooves are disposed between electrode fingers of an IDT or between conductor strips of a reflector, which is described in JP-A-57-5418 and “Manufacturing Conditions and Characteristics of Groove-type SAW Resonator”, Technological Research Report of the Institute of Electronics and Communication Engineers of Japan MW82-59 (1982). The “Manufacturing Conditions and Characteristics of Groove-type SAW Resonator” also discloses that the frequency-temperature characteristic varies depending on the depth of the grooves.
Japanese Patent No. 3851336 discloses that a configuration for setting a curve representing the frequency-temperature characteristic to a three-dimensional curve is used in the SAW device employing an LST-cut quartz crystal substrate and that any substrate with a cut angle having a temperature characteristic represented by a three-dimensional curve could not be discovered in a SAW device employing Rayleigh waves.
As described above, there exist various factors for improving the frequency-temperature characteristic. Particularly, in the SAW device employing the Rayleigh waves, the increase in thickness of an electrode of an IDT is considered as a factor contributing to the frequency-temperature characteristic. However, the applicant of the invention experimentally discovered that the environment resistance characteristic such as a temporal variation characteristic or a temperature impact resistance characteristic is deteriorated by increasing the thickness of the electrode. For the main purpose of improvement in the frequency-temperature characteristic, the thickness of the electrode should be increased as described above, and it is thus difficult to avoid the deterioration in the temporal variation characteristic or the temperature impact resistance characteristic. This is true of a Q value and it is difficult to increase the Q value without increasing the thickness of the electrode.